The Mw=7.9 Denali earthquake A Mw = 7.9 earthquake occurred on November 3, 2002, on the Denali fault in Alaska. It was preceded 12 days earlier by a Mw = 6.7 foreshock, immediately to the west of the main rupture. The Denali earthquake is the largest strike-slip rupture in the US since 1857. The Denali earthquake ruptured about 300 km along the Denali fault system, a major crustal discontinuity that extends for more than 2000 km across Alaska and northwestern Canada. Estimates of average right-lateral Holocene slip rates range from about 1 cm/yr to about 3.5 cm/yr. GPS measurements of interseismic deformation indicate about 1 cm/yr of right-lateral strain accumulation across the central segment of the Denali fault zone.

Document Univ. of Alaska, Fairbanks

Surface rupture of the Denali earthquake

Postseismic deformation following the Denali earthquake The postseismic response of the lithosphere to the Denali earthquake is likely to affect a ~300 km wide area on both sides of the rupture for several years. Monitoring this postseismic deformation with GPS, in particular with continuous measurements, will enable us to better understand (1) the dominant post-seismic deformation mechanisms in the Earth's lithosphere, (2) the rheological parameters of the fault zone and surrounding crust and upper mantle, and (3) the physics of fault interaction. The large moment of the earthquake, the precise pre-earthquake deformation field, and the immediate field response make this the most promising target to address postseismic deformation problems since modern space-geodetic methods became available.

GPS observation strategy Our observational strategy consists of continuous GPS (CGPS) measurements at 16 sites located at a wide range of distances from the rupture, including some at > 150 km, in order to distinguish processes in the deep fault zone, the lower crust and upper mantle. Eleven of these sites were installed within two weeks after the event. Six additional sites were installed in the Summer 2003. A fundamental objective of the proposed research is to precisely determine the time-dependence of the postseismic deformation signal. The early postseismic response (Nov. 2002) and the upgrade and installation of new CGPS sites was made possible thanks to the UNAVCO Alaska earthquake response.

Continuous GPS and UNAVCO field engineers

Map of Denali continous and campaign GPS sites.

GPS network CGPS measurements (squares on the map on the right) are complemented by two GPS campaigns per year using existing benchmarks (triangles on the map on the right), in order to spatially densify the postseismic displacement field. GPS campaigns are performed in May-June and September-October.

Deformation modeling The observational efforts are complemented by comprehensive modeling efforts to evaluate candidate deformation mechanisms such as velocity-strengthening afterslip, poroelastic rebound, and viscous flow in the lower crust and upper mantle. Characterization of the relative importance of each of these mechanisms is a major goal of this research project. We will fully explore the ability of the different model types (e.g., time-dependent afterslip on and below the rupture vs. powerlaw flow in the lower crust and upper mantle) to fit the data while being consistent with complementary geological and geophysical constraints.

Postseismic deformation models (A. Freed)

See also:

• The USGS Denali earthquake web site.
• Denali earthquake response, UNAVCO.
• Denali earthquake response, UNAVCO.
• Pictures of the continous sites, S. Hreinsdottir, UAF.
• Univ. of Alaska, Fairbanks
• USGS News
• Stress transfer, G. Anderson (USGS)
• Misc